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Manufacturing method of metal injection molding raw material

Categories: ZCMIM NewsStars: 3StarsVisit: -Release time: 2019-01-17 10:18:00
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Metal powders are groups of metallic particles with a size less than 1mm.It includes single metal powder, alloy powder and some refractory compound powder with metal properties. It is main raw material for metal injection molding and powder metallurgy.

The properties of metal powder are largely determined by production method and preparation process. Basic properties of powders can be determined by specific standard test methods. There are many methods to determine particle size and distribution of powder, such as sieve analysis (>44μm), sedimentation analysis (0.5 ~ 100μm), gas transmission method, microscope method and so on. Ultrafine powder (<0.5μm) were determined by electron microscopy and X-ray small-angle scattering. On usual, metal powders are classified as five grades: coarse powder, medium powder, fine powder, minuteness powder and ultrafine powder. Generally, according to principle of transformation, it can be divided into mechanical method and physicochemical method, which can be directly refined from solid, liquid and gaseous metals, and can be made from metal compounds in different states through reduction, pyrolysis and electrolysis. The carbides, nitrides, borides and silicides of refractory metals can be prepared directly by chemical combination or reduction-chemical combination. Due to different preparation methods, the shape, structure and particle size of same powder often vary greatly. Preparation methods of powder are listed as follows, among which most widely used are reduction method, atomization method and electrolysis method.

Atomization method: atomized molten metal is atomized into tiny droplets, solidified it to powder in cooling medium. The widely used second-stream (melt flow and high-speed fluid medium) atomization method is to use high-pressure air, nitrogen, argon (gas atomization) and high-pressure water (water atomization) as injection medium to smash the metal liquid. There are also centrifugal atomization methods that grinding and rotating melt itself (consumable electrode and crucible) by rotating disc, as well as other atomization methods such as vacuum atomization of dissolved hydrogen and ultrasonic atomization. Due to fine droplets and great heat exchange conditions, the condensation rate of droplets can reach 100 ~ 10000K/s, several orders of magnitude higher than that of ingot casting. Therefore, composition of the alloy is uniform and microstructure is fine. Generally, aerosol powder is nearly spherical, and water atomization can produce irregular shape. The characteristics of powder, such as particle size, shape and crystal structure, mainly depend on melt properties (viscosity, surface tension, super heat) and atomization process parameters (such as melt flow diameter, nozzle structure, jet medium pressure, velocity, etc.).Almost all metals that can be melted can produced by atomization, especially alloy powders. This method has high production efficiency and is easy to expand industrial scale. At present, it is not only used for mass production of iron, copper, aluminum powder and various kinds of alloy powder for industrial use, but also used to produce high-purity (O2<100ppm) high-temperature alloy, high-speed steel, stainless steel and titanium alloy powder. In addition, the rapid condensation powder ( condensation rate is >100,000K/s) is increasingly be valued by using the chilling technology, which can be used to produce high-performance microcrystalline materials.

Mechanical Comminution: This process breaking solid metals into powder by crushing and grinding. The equipment is divided into coarse crushing and fine crushing. Such as hammer crusher, rod mill, ball mill, vibratory mill and agitation ball mill and other fine grinding equipment, mechanical comminution is mainly suitable for crushing metals and alloys that brittle and easy work hardening, such as tin, manganese, chromium, iron with high carbon content, iron alloy, etc., are also used to broken reduction method of spongy metal, electrolytic law of cathode deposit; It is also used to crush hydrogenate brittle titanium and then dehydrogenate it to make fine titanium powder. Mechanical comminution has low efficiency and high energy consumption. It is often used as a supplement to other methods of making powder, or for mixing powder with different properties. In addition, mechanical comminution method also includes vortex grinding machine, which depends on two impellers to create eddy current, so that particles enclosed by airflow collide with each other at high speed and are crushed, which can be used for the crushing of plastic metals. Cold flow crushing is a process in which high speed and high pressure inert gas with coarse powder is sprayed onto a metal target. Due to adiabatic expansion in the nozzle exit airflow, temperature drop below 0 ℃, make with low temperature brittleness of metal and alloy thick powder grinding into fine powder. Mechanical alloying is grinding of different metals and compounds with high melting point into solid solution or fine dispersion alloy state by high energy ball mill.

Reduction: This process by which a reducing agent captures the oxygen in a metal oxide powder so that metal is reduced to powder. Gas reducing agent has hydrogen, ammonia, gas, change natural gas. Solid reducing agent include carbon ,sodium, calcium and magnesium. Hydrogen or ammonia reducers are used to produce metallic powders such as tungsten, molybdenum, iron, copper, nickel and cobalt. With metal strong reducing agent sodium, magnesium, calcium, can produce tantalum, niobium, titanium, zirconium, vanadium, beryllium, thorium, uranium and other metal powder. Nickel, copper, cobalt and their alloys or coated composite powders can be prepared by reducing metal salt aqueous solution with high pressure hydrogen. Most of powder particles produced by reduction method have irregular shape of sponge structure. The particle size of powder mainly depends on reduction temperature, time and particle size of raw material. Reduction method which can be used to prepare powder of most kinds of metal, is a widely used method.

Electrolysis method: in metallic saline solution by direct current, metal ions are discharge precipitate on the cathode, easy to break into powder sedimentary layer. Metal ions generally come from the dissolution of the same metal anode and migrate from the anode to the cathode under the action of current. The main factors affecting powder size are composition of electrolyte and electrolytic conditions. Generally, electrolytic powder is dendritic with high purity, but this method consumes a lot of electricity and has high cost. Electrolysis is also widely used in production of copper, nickel, iron, silver, tin, lead, chromium, manganese and other metal powders. Alloy powder can also be made under certain conditions. For rare refractory metals such as tantalum, niobium, titanium, zirconium, beryllium, thorium and uranium, composite molten salt is often used as electrolyte to prepare powder.

Carbonyl method: synthesis of certain metals (iron, nickel, etc.) with carbon monoxide into carbonyl compounds, and then thermal decomposition into metal powder and carbon monoxide. Resulting powder is very fine (0.0005 to a few microns in size) and very pure but also costly. In industry, it is mainly used to produce fine powder and ultrafine powder of nickel and iron, and alloy powder of Fe-Ni、Fe-Co、Ni-Co.

Direct method: carbon, nitrogen, boron and silicon are directly combined with refractory metals at high temperature. The reduction - combination method use carbon, nitrogen, boron carbide, silicon and refractory metal oxides. Both methods are commonly used for the production of carbide, nitride, boride and silicide powders.

With the improve of technology, metal powder has shown a good application prospect in metallurgy, chemical industry, electronics, magnetic materials, fine ceramics, sensors and other aspects. However, the limitation of traditional preparation technology restricts application of metal powder. Although many new production processes and methods have been applied, the problems of small scale and high cost are still not well solved. In order to promote development of metal powder materials, it is necessary to strengthen innovation, learn from each other and develop a production process with larger output and lower cost.

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